This invention relates generally to a packaging machine and more particularly to a packaging machine of the type which is known as a vertical form, fill and seal machine. In such a machine, a web of packaging material (e.g., a thin film) is drawn from a supply roll and is pulled past a forming collar so as to shape the web into a tube which then is sealed longitudinally. The tube also is sealed transversely at package length intervals and then is cut into individual packages, product being dropped into the tube between successive transverse sealing operations.
In order to form packages of proper length, it is necessary that a precisely predetermined length of the film be drawn past the forming collar during the time interval which occurs between successive transverse sealing operations. To insure against the film being severely stretched or stressed, it is desirable to pull the film past the forming collar with only sufficient tension to maintain the web taut during the forming, filling and sealing operations.
Various machines have been devised for drawing a predetermined length of film from the supply roll for each transverse sealing operation while still maintaining the film under low tension as it is pulled past the forming collar. In virtually all of such machines, a power assist pay off mechanism is located upstream of the forming collar to feed the film from the supply roll to the collar while a separate pull down mechanism is located downstream of the forming collar to pull the film past the collar with low tension. For example, Henry U.S. Pat. No. 3,921,928 discloses a machine in which the supply roll itself is power-rotated to feed the film toward the collar while vertically reciprocable transverse seal bars draw a predetermined length of film past the collar. In the machine disclosed in James U.S. Pat. No. 4,288,965, coacting feed rolls located upstream of the forming collar pull a predetermined length of film from the supply roll and feed that length toward the collar. The film then is pulled past the collar by endless belts which frictionally engage the film.
Other packaging machines with a power assist pay off mechanism upstream of the forming means and with a separate pull down mechanism downstream of the forming means are disclosed in Gausman U.S. Pat. No. 3,449,888; Hudson et al U.S. Pat. No. 3,466,850; Egger U.S. Pat. No. 3,789,569; Greenawalt et al U.S. Pat. No. 4,009,551 and Ogata U.S. Pat. No. 4,144,693.
In machines of the above type, it is necessary to correlate in some manner the action of the upstream power assist mechanism and the action of the downstream pull down mechanism. For example, in the machine disclosed in the James patent, the downstream pull down belts are overdriven with respect to the upstream feed rolls and slip relative to the newly formed tube in order to insure that the belts will pull down all of the film fed out by the rolls without exerting excessive tension on the film. The slippage causes the belts to wear at a comparatively rapid rate and, if a belt breaks or the downstream drive otherwise malfunctions, the upstream feed rolls will continue to deliver film to the forming collar until such time as the entire machine is shut down. In other machines (e.g., the machine disclosed in the Henry patent), a vertically movable dancer roll is located between the power assist mechanism and the forming means to correlate the action of the power assist mechanism with the action of the pull down mechanism. With most machines of this type, the dancer roll causes the power assist mechanism to cycle intermittently and somewhat separately of the pull down mechanism. As a result, the correlation between the two mechanisms is less than optimum and, in addition, it is difficult to maintain good control over the film tension.